Spacecraft Behaving Badly

Could strange new physics be affecting a pair of distant space probes?

Because the flow of heat energy in various directions call have un-expected effects, one of the things Anderson and Turyshev looked at was the spacecraft's material self-specifically the way heat would be absorbed, conducted, and radiated from one surface to another. Their inquiry managed to account for about a tenth of the anomaly. But neither investigator is a thermal engineer. A wise second step: find one. So in early 2006 Turyshev sought out Gary Kinsella, a JPL colleague who until that moment had never met either him or a Pioneer face to face, and convinced Kinsella to take the thermal issues to the next level. Last spring, all three men came to the Hayden Planetarium in New York City to tell a sellout crowd about their still-unfinished travails. Meanwhile, other researchers worldwide have been taking up the challenge too.

Consider what it's like to be a spacecraft living and working hundreds of millions of miles from the Sun. First of all, your sunny side warms up while the unheated hardware on your shady side call plunge to 455 degrees below zero Fahrenheit, the background temperature of outer space. Next, you're constructed of many different kinds of materials and have multiple appendages, all of which have different thermal properties and thus absorb, conduct, emit, and scatter heat differently, both within your various cavities and outside to space. In addition, your parts like to operate at very different temperatures: your cryogenic science instruments do fine in the frigidity of outer space, but your cameras favor room temperature, and your rocket thrusters, when fired, register 2,000 degrees F. Not only that, every piece of your hardware sits within ten feet of all your other pieces of hardware.

The task facing Kinsella and his team of engineers was to assess and quantify the directional thermal influence of every feature on board Pioneer 10. To do that, they created a computer model representing the spacecraft surrounded by a spherical envelope. Then they subdivided that surface into 2,600 zones, enabling them to track the flow of heat from every spot in the spacecraft to and through every spot in the surrounding sphere. To strengthen their case, they also hunted through all available project documents and data files, many of which hail from the days when computers relied on punch cards for data entry and stored data on nine-track tape. Without emergency funds from the Planetary Society, by the way, those irreplaceable archives would shortly have ended up in a dumpster.

For the simulated world of the team's computer model, the spacecraft was placed at a test distance from the Sun (25 AU) and at a specific angle to the Sun, and all the parts were presumed to be working as they were supposed to. Kinsella and his crew determined that, indeed, the uneven thermal emission from the spacecraft's exterior surfaces does create an anomaly—and that it is indeed a continuous, sunward change in velocity.

But how much of the Pioneer anomaly can be chalked up to this thermal anomaly? Some. Perhaps even most. But not all. The team's thermal model was based solely on the trajectory and hardware data from Pioneer 10, which displays a smaller anomaly than that of Pioneer 11. Not only that, the researchers have yet to calculate how the thermal anomaly varies with Pioneer 10's (let alone Pioneer 11's) distance from the Sun.

So what about the as-yet-unexplained "not all" portion? Do we sweep it under the cosmic rug in hopes that additional Kinsellan analysis will eventually resolve the entire anomaly? Or do we carefully reconsider the accuracy and inclusiveness of Newton's laws of gravity, as a few zealous physicists have been doing for a couple of decades?

Pre-Pioneers, Newtonian gravity had never been measured—and was therefore never confirmed—with great precision over great distances. In fact, Slava Turyshev, an expert in Einstein's general relativity, regards the Pioneers as (unintentionally) the largest-ever gravitational experiment to confirm whether Newtonian gravity is fully valid in the outer solar system. That experiment, he contends, shows it is not. As any physicist can demonstrate, beyond 15 AU the effects of Einsteinian gravity are negligible. So, at the moment, two forces seem to be at play in deep space: Newton's laws of gravity and the mysterious Pioneer anomaly. Until the anomaly is thoroughly accounted for by misbehaving hardware, and can therefore be eliminated from consideration, Newton's laws will remain unconfirmed. And there might be a rug somewhere in the cosmos with a new law of physics under it just waiting to be uncovered.